I'm recording this in case someone else hits my problem.

I developed an ASP.net Ajax enabled module.  It worked fine on my development environment, but not when uploaded to my server.  After much messing about I discovered that in my environment DNN 4.09.00 it would work correctly ONLY when using the MinimalExtropy skin.  On my server (at that time still using DNN 4.08.02 and DNN_Blue) it failed.

My specific failure points were that the Calendar Extender would display the calendar at the top of the window - should be below the extended control, and the ListSearchExtender which displayed it's prompt at the top of the window instead of near the extended control.

I upgraded the server to DNN 4.09.01 - no change.  The fix for me was to use the MinimalExtropy skin.  It changes the look of the site, but I'm happy with the new look.

It is perfectly possible that I screwed up my code in some way (are there known tar-pits in this area) but until more evidence emerges; I think it's the skin.  Extensive Googling failed to turn up any similar experiences in this area so I am fully prepared to believe it's something I did.  I'd welcome any suggestions.

actually the problem is that a number of the ms ajax (and toolkit) components require xhtml compliance to work correctly. Both of the extropy skins are xhtml compliance , whereas older skins such as dnn_blue use html 4 as their doctype. You can override the doctype either via a file (http://www.dotnetnuke.com/Community/Blogs/tabid/825/EntryId/1226/using-Skin-level-DocTypes.aspx) or else if you're using 4.9 or above, via the UI http://www.dotnetnuke.com/Community/Blogs/tabid/825/EntryId/1905/DotNetNuke-5-0-xhtml-compliance-Part-2.aspx

Cathal